Studies will be made to determine the nature of the myometrial inhibitory factors discovered in the circulating blood of pregnant rabbits and guinea pigs. Two approaches will be employed. In the guinea pig evidence suggests the factor may be relaxin-like so that immunological methods will be employed. Antisera raised to porcine relaxin will be used to obtain evidence as to whether or not the factor is relaxin through immunoprecipitation methods and direct challenges to pregnancy maintenance. The action of relaxin will be studied in vivo, in vitro, in situ using intra-uterine pressure and suction electrode recording techniques to determine in what manner it affects electrical activity and pharmacological reactivity (eg to oxytocin, catecholamines and prostaglandins). The action of progesterone on the myometrium of the rat in vivo in situ will also be investigated since pilot studies have revealed a surprising lack of response in postpartum animals.